1. Field of the Invention
The present invention relates to a beam homogenizer for homogenizing energy distribution of a beam spot on an irradiation surface in a particular region. Further, the present invention relates to a laser irradiation apparatus equipped with a laser oscillator and the beam homogenizer, wherein a laser beam emitted from the laser oscillator is homogenized through the beam homogenizer and wherein an anneal process and the like are conducted by irradiating the irradiation surface with the beam spot. Furthermore, the present invention relates to a method for manufacturing a semiconductor device using the laser irradiation apparatus. It is to be noted that the semiconductor device includes a liquid crystal display device, a light-emitting device using electroluminescence (EL), an electronic optical device, and an electric appliance.
2. Related Art
A process of manufacturing a semiconductor device includes a step of crystallizing a semiconductor film over a glass substrate. The semiconductor film is crystallized by, for example, laser annealing. Since the laser annealing can give energy only to the semiconductor film locally, the laser annealing has an advantage of preventing softening of glass which has been a problem in annealing by conventional heat treatment.
In recent years, a laser oscillator having high output power has been employed with the increase in size of a substrate. A method is widely employed in which a laser beam with high output power typified by an excimer laser or the like is shaped into a linear beam spot on an irradiation surface by an optical system and an irradiation position of the beam spot is moved relative to the irradiation surface. With this method, high mass productivity can be obtained.
At the irradiation with the linear beam spot, the linear beam spot is delivered while being overlapped by displacing the beam spot gradually in a direction perpendicular to a line direction of the beam spot (the direction perpendicular to the line direction is hereinafter referred to as a width direction). By the irradiation while overlapping the beam spots, the growth of a crystal grain in the semiconductor film can be promoted and the characteristic such as electron mobility can be enhanced.
In the case of conducting the laser annealing of the semiconductor film, it is necessary to set the energy distribution and the energy density of the linear beam spot in the width direction to an optimum condition for the laser annealing. Furthermore, to conduct the laser annealing homogeneously to the semiconductor film, the fluctuation in the energy distribution and the energy density of the linear beam spot formed by the optical system needs to be suppressed. However, in the case of using a homogenizer including a cylindrical lens array, the position of a plane at which a homogeneous linear beam spot is obtained changes when a beam parameter such as a divergence angle fluctuates due to the change of a laser oscillation condition. As a result, a defect is found in which the energy distribution or the energy density of the linear beam spot on the irradiation surface fluctuates. Consequently, a beam homogenizer using an optical waveguide instead of the cylindrical lens array and a laser annealing apparatus having such a beam homogenizer mounted have been suggested (see, for example, Japanese Patent Application Laid-Open No. 2004-134785).